Add CODEX repository review document

[adrianwedd]

Motivation

• Provide a single, navigable repository review summarizing health, security, and operational recommendations.
• Surface critical issues (auth defaults, session backend inconsistencies, documentation gaps) so maintainers can triage quickly.
• Create an artifact that documents findings, latent risks, and questions for maintainers in one place.

Description

• Added CODEX-REVIEW.md at the repository root containing an Executive Summary, Critical Issues, Priority Improvements, Latent Risks, Questions for the Maintainer, and a What’s Good section.
• The review cites concrete files and code paths (for example sdb/config.py, sdb/ui/app.py, and sdb/ui/redis_session_backend.py) and recommends fixes and follow-ups.
• No production code was changed; this PR only introduces the new documentation file.

Testing

• Ran the test suite with pytest -q, which failed during collection due to missing development dependencies (notably httpx and typer).
• The pytest run produced import errors (47 errors during collection) preventing a full test pass.
• No other automated test changes were made as part of this PR.

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Codex Task